1. Field of the Invention
This invention relates to an integrated semiconductor laser-modulator device and a manufacturing process of the same. More particularly, it relates to the modulator-integrated semiconductor laser device in which frequency-characteristics have been improved by suppressing an influence of the fluctuation of the electric field due to the modulating signals applied to the modulator, and a manufacturing process thereof.
2. Description of Related Art
In order to popularize the public communication web with optical fibers, it is important to improve the performance of such semiconductor laser devices and the productivity for inexpensively manufacturing the same.
Especially for improving the performance of semiconductor laser devices, it is essential to the laser beam thereof at high frequency in order to transmit an increasing amount of information. Since the fluctuation of wavelength modulation should be minimized to transmit the laser beam a long distance, an external modulation system is generally adapted for modulating the laser beam. A modulator device switches on and off beam transmittance, thereby modulating the laser beam at high frequency, while the semiconductor laser device keeps emitting at a constant level.
Since the external modulation system has difficulties in coupling the modulator with the semiconductor laser beam device and in requiring a lot of components, an integrated semiconductor laser-modulator devices, which are a semiconductor laser device monolithically integrated with a modulator, have been developed to overcome such difficulties.
According to such integrated modulator semiconductor laser device, while is common electrode are connected to a ground, the forward biased current is injected into the semiconductor laser and a reverse biased modulating signals are applied to the modulator. Therefore the structure of a separator region provided between the semiconductor laser and the modulator becomes quite important.
Since a high extinction ratio (i.e. a ratio of beam transmittances between ON-state and OFF-state) can be achieved with a low operating voltage when the semiconductor modulator has a beam absorbing multi quantum well (referred to as MQW hereinafter) structure layer, the MQW beam absorption layer is typically adapted in high rate communicating.
FIG. 27 shows a partially sectional perspective view of a conventional modulator-integrated semiconductor laser device (referred to as a modulator-integrated laser device). FIG. 28 shows a sectional view taken along lines XXVIIIxe2x80x94XXVIII of FIG. 27.
In FIG. 27 and FIG. 28, numeral 201 denotes a modulator-integrated laser device, the D region represents a separator/modulator region consisting of a separator region and a modulator region, and the C region represents a laser region.
In FIG. 27 and FIG. 28, numeral 202 denotes a substrate of InP, numeral 204 denotes a laser n-side beam confinement layer of InGaAsP, 206 denotes a MQW active layer of InGaAsP, 208 denotes a laser p-side beam confinement layer of InGaAsP. And a laser waveguide 207 is comprised with the laser n-side beam confinement layer 204, the active layer 206, and the laser p-side beam confinement layer 208.
Numeral 210 denotes a first clad layer of p-InP, numeral 212 denotes a diffractive grating, numeral 214 denotes a second clad layer of p-InP, numeral 216 denotes a contact layer of p+-InGaAs. xe2x80x9cn-xe2x80x9d is referred to xe2x80x9cn-typexe2x80x9d, and xe2x80x9cp-xe2x80x9d is referred to xe2x80x9cp-typexe2x80x9d hereinafter.
Numeral 218 denotes an n-side laser beam confinement layer of n-InGaAsP in the separator/modulator region, numeral 220 denotes a MQW beam absorption layer of InGaAsP in the separator/modulator region consisting of a beam absorption layer 220a in the modulator region and a beam absorption layer 220b in the separator region. Numeral 222 denotes a p-side beam confinement layer of InGaAsP in the separator/modulator region. A waveguide 221 in the separator/modulator region is composed with the n-side beam confinement layer 218 in the separator/modulator region, the beam absorption layer 220, and the p-side beam confinement layer 221 in the separator/modulator region.
Numeral 224 denotes a first embedded layer of InP doped with Fe, numeral 226 denotes a Hall-trap layer of n-InP, and numeral 228 denotes a second embedded layer of InP doped with Fe. The ridge-shaped laser waveguide 207 and the ridge-shaped separator/modulator waveguide 221 have both side surfaces along the laser emitting direction on which the first embedded layer 224, the Hall-trap layer 226, and the second embedded layer 228 are composed to form a current block layer. A window structure 229 is formed at the emitting end surface of the current block layer.
Numeral 230 denotes an insulator layer of SiO2, numeral 231 denotes a separator groove separating the laser region C from the modulator region. Numeral 232 denotes a surface evaporated electrode of Ti/Au. Numeral 234 denotes a p-side laser Au-coated electrode. Numeral 236 denotes a p-side modulator Au-coated electrode. Numeral 232 denotes a surface evaporated electrode. Numeral 240 denotes a common electrode. And numeral 242 denotes an arrow showing the direction of the emitting laser beam.
A conventional modulator-integrated laser device is manufactured as described below.
First by using a MOCVD method, formed on the substrate 12 of n-InP are in sequence, an n-InGaAsP layer of the n-side laser beam confinement layer 204, an InGaAsP MQW layer of the active layer 206, a p-InGaAsP layer of the p-side laser beam confinement layer 208, a p-InP layer of the first clad layer 210, and P-InGaAsP layer for forming the diffractive grating 212.
Next, by using an interference exposing method, the P-InGaAsP layer is etched with a grating shape to form the diffractive grating 212. Thereafter a P-InP layer of the first clad layer 210 is disposed entirely to embed the diffractive grating 212.
A dielectric layer of material such as SiO2 and SiN is formed on the p-InP layer of the first clad layer 210, and etched to form a stripe-shaped dielectric layer that includes the diffractive gratings 212 and forms the laser waveguide. Then layers disposed in the region including the separator/modulator region D are etched with using the dielectric layer as a mask until the substrate 202 is exposed.
Next, while retaining the stripe-shaped dielectric layer, by using a MOCVD method, subsequently formed are, the n-InGaAsP of the n-side laser beam confinement layer 218 in the separator/modulator region, the MQW layer of InGaAsP of the beam absorption layer 220 in the separator/modulator region D, and the InGaAsP layer of the p-side beam absorption layer 222 in the separator/modulator region.
Next, after the stripe-shaped dielectric layer is removed, again an another dielectric layer of material such as SiO2 and SiN is deposited to form the waveguide in the separator/modulator region D and the laser region C, and etched to make the stripe-shaped dielectric layer with stripe-shaped which overlaps over the separator/modulator region D and the laser region C. Then the entire layers are wet-etched with HBr (hydrogen bromide) using a mask of this dielectric layer until the substrate 202 is exposed so that the waveguide is shaped as a ridge. When the window structure 229 is formed, the stripe-shaped dielectric layer is such that the layers at the emitting end surface can be also etched away.
Next, while retaining the stripe-shaped dielectric layer, the InP layer doped with Fe of the first embedded layer 224, the n-InP layer of the Hall-trap layer 226, and the InP layer doped with Fe of the second embedded layer 228 are grown by using the MOCVD method.
After removing the stripe-shaped dielectric layer, a p-InP layer of a second clad layer 214 and a p+-InGaAs of a contact layer 216 are grown entirely by using the MOCVD method.
Next, the contact layer 216 and the second clad layer 214 in a region corresponding to separator region are partially etched to form a separator groove 231.
After an insulator layer 230 is entirely deposited by sputtering and etched away at the contact electrode in the modulator region and the laser region C, the Ti/Au layer and the Au-coated layer are formed on the surface evaporated electrode 232.
After a backside of the substrate 202 is thinned so that the substrate is approximately 100 micrometer thick, a sequence of AuGe/Ni/Ti/Pt/Ti/Pt/Au layer of the evaporated electrode is formed by depositing, and the Au-coated layer is patterned to form the common electrode 240 thereon.
According to the modular-integrated semiconductor laser device 201 formed as described above, the forward biased voltage are applied between the laser electrode 234 and the common electrode 240 to inject the current into the active layer 206, so that the laser beam emitted in the laser region C is guided through the separator/modulator waveguide 221 into the separator/modulator region D. And the modulating signals of reverse biased voltage are applied between the modulator electrode 236 and the common electrode 240 to provide the beam absorption layer 220a with an electric field corresponding to the modulating signals, so that the laser beam 242 is modulated and emitted at high frequency and at high extinction rate because of the Stark quantum confinement effect.
The conventional modular-integrated laser device 201 is composed as described above, the p-side laser electrode 234 is spaced away through the separator groove 231 and the insulator layer 230 provided thereon from the p-side modulator electrode 236 to prevent the wavelength of the laser beam 242 from fluctuating.
However because the beam absorption layer 220a of the modulator is formed of MQW, while the extinction rate is advantageously high at the low voltage, the refractive index tends to fluctuate due to the unstable electric field caused by the modulating signals. As the result, the fluctuation of the wavelength because of the unstable refractive index can not be always suppressed. When the wavelength fluctuates too much, it is impossible to transmit the beam especially in long distance.
The laser region C is connected through a composition face to the separator/modulator region D, which is formed by re-growing. Thus the separator/modulator region D is formed by a single butt-joint method to have a structure with even bandgap.
In operating such modulator-integrated semiconductor laser devices 201, the modulating signals are applied between the modulator electrode 236 and the common electrode 240. The electric field generated by the modulating signals arises not only between the modulator electrode 236 and the common electrode beneath the modulator electrode 236, but also between the modulator electrode 236 and the common electrode in the laser region C. Thus the electric field arises beneath the separator groove 231 in the separator/modulator region 221, which is referred to as a xe2x80x9cleakage electric fieldxe2x80x9d hereinafter.
As described, the beam waveguide layer 220b and the beam absorption layer 220a are formed by a single butt-joint method to have a structure with even bandgap. Therefor the emitted beam is absorbed responding to the electric field caused not only by the modulating signals in the beam absorption layer 220a beneath the modulator electrode, but also by a minor electric field derived in the beam waveguide layer 220b beneath the separator groove 231, which acts as a beam absorption layer. This is the problem to be solved.
If the beam waveguide layer 220b acts as the beam absorption layer, since the electric field in the beam waveguide 220b is less than that in the absorption layer 220a, the size of the depletion layer at the p-n junction of the beam waveguide layer 220b does not expand so that the junction capacitance increases. Consequently since the beam is absorbed much in low frequency area and little in high frequency of the modulating signals, the entire frequency-characteristics sometimes shows not flat and distorted with an undulant shape.
A conventional example is described in the Japanese Laid-Open publication of JP 8-335745A, showing a modulator-integrated semiconductor laser device, comprising waveguides in the laser region, the separator region, and the modulator region, of which bandgap wavelength are different.
According to the publication, the modulator-integrated semiconductor laser device formed by the butt-joint method is designed to shorten the wavelength by implanting ion into the separator region. However, it does not disclose that the beam absorption layer in the modulator region is formed of the bulk structure, nor does that the separator region and separator region are disposed through the composition face, that is different from the structure described hereinafter according to the present invention.
An another conventional example is described in the Japanese Laid-Open publication of JP 7-176827A showing a modulator-integrated semiconductor laser device, comprising waveguides in the semiconductor laser region, the modulator region, and the transition region, which are simultaneously disposed by the selective growth method. Further it discloses that the emitting wavelength in the semiconductor laser region is greater than that of the modulator region, and that the emitting wavelength in the semiconductor laser region and the modulator region is greater than that in the transition region intervening between both regions. But it does not disclose that the laser region are formed and aligned by the butt-joint method nor does that the active layer, the beam waveguide layer, and the beam absorption layer compose the MQW structure, that is different from the structure described hereinafter according to the present invention.
This invention is to resolve the above-mentioned problems, and the first object is to provide a modulator-integrated semiconductor laser device, in which a modulator is integrated with a semiconductor laser device, having an improved frequency-characteristics by reducing the influence of the fluctuating electric field generated by the modulator signals applied to the modulator.
The second object is to provide the modulator-integrated semiconductor laser device including the modulator of a bulk structure to suppress the fluctuation of the beam absorption ratio caused by the electric field of the modulating signals thereby reducing the fluctuation of the refractive index due to the fluctuation of the beam absorption ratio, and also the laser device including the beam waveguide with a bulk structure in the separator region between the semiconductor laser and the modulator to suppress the beam absorption in the beam waveguide due to the leakage electric field. As the result, the laser device performs a good frequency-characteristics and high power output.
The third object is to provide the modulator-integrated semiconductor laser device, comprising the semiconductor laser region, the modulator region, and separator region therebetween, wherein the waveguides in each region are one another connected and aligned through the composition faces, and also comprising a beam waveguide of bulk structure in the separator region to suppress the influence of the fluctuating electric field caused by the modulating signals applied to the modulator thereby improving the frequency-characteristics thereof.
The forth object is to provide a process for easily manufacturing the modulator-integrated semiconductor laser device, in which the frequency-characteristics is improved.
A modulator-integrated semiconductor laser device according to the present invention comprises a semiconductor substrate of a first conductive type, and the substrate has a first and second main surface. The device has a first ridge-shaped waveguide disposed on a part of the first main surface of the semiconductor substrate, and the first waveguide has an active layer and a longitudinal direction. The device also has a second ridge-shaped waveguide formed on the semiconductor substrate extending along the longitudinal direction to connect to the first waveguide, and the second waveguide includes a beam waveguide layer of a bulk structure of bandgap greater than that of the active layer. The device includes a third ridge-shaped waveguide formed on the semiconductor substrate extending along the longitudinal direction to connect to the second waveguide. And the third waveguide has a beam absorption layer of a bulk structure of bandgap greater than that of the active layer but less than that of the beam waveguide layer. Further the device has a first ridge-shaped clad layer of a second conductive type, deposited on the first, second, and third waveguides. The device includes a second ridge-shaped clad layer sandwiched between either one of the first clad layer or the substrate on one hand, and the first, second and third waveguides on the other hand. A diffractive grating is formed and embedded within the second clad layer corresponding to the first waveguide. A current block layer is deposited on the semiconductor substrate positioned on both sides of the longitudinal direction along the first and second clad layer and the first, second, and third waveguides. A first electrode is deposited over the first clad layer opposing to the active layer and a second electrode spaced from the first electrode and deposited over the first clad layer opposing to the beam absorption layer. Also a third electrode is deposited on the second main surface of the semiconductor substrate. The device formed as described above is improved in the frequency-characteristics by suppressing the influence of the unstable electric field due to the modulating signals applied to the modulator, in other words by stabilizing the fluctuating absorption ratio caused by the electric field of the modulating signals to minimize the fluctuation of the refractive index due to the fluctuating absorption ratio, and also by forming the beam waveguide of a bulk structure in the separator region sandwiched between semiconductor laser region and the modulator region to suppress the beam absorption in the beam waveguide due to the leakage electric field.
Further the second waveguide is connected to the first waveguide through a first composition face, so that high design flexibility is permitted while keeping the frequency-characteristics improved.
Further the third waveguide is connected to the second waveguide through a second composition face and wherein the beam waveguide layer and the beam absorption layer are formed of a bulk crystal structure, therefore the beam absorption is reduced and the frequency characteristics is improved.
Also both of the waveguide layer and the beam absorption layer are formed by compositionally disordering, and each of the waveguide layers has a multi quantum well structure, and a compositional disordering ratio of the beam waveguide layer is higher than that of the beam absorption layer, so that the frequency-characteristics is improved with a simple structure.
Also the beam waveguide layer of the second waveguide is thinner towards the first waveguide and thicker towards the third waveguide, so that the beam absorption is reduced and the frequency characteristics is improved.
A modulator-integrated semiconductor laser device comprises a semiconductor substrate of a first conductive type, and the substrate has a first and second main surfaces. The device has a first ridge-shaped waveguide deposited on a part of the first main surface of the semiconductor substrate, and the first waveguide has an active layer and a longitudinal direction. The device also has a second ridge-shaped waveguide formed on the semiconductor substrate extending along the longitudinal direction to connect to the first waveguide through a first composition face. And the second waveguide includes a beam waveguide layer of bandgap greater than that of the active layer. A third ridge-shaped waveguide is formed on the semiconductor substrate extending along the longitudinal direction to connect to the second waveguide through a second composition face. The third waveguide includes a beam absorption layer of a multi quantum well structure of bandgap greater than that of the active layer but less than that of the beam waveguide layer. A first ridge-shaped clad layer of a second conductive type, is deposited on the first, second, and third waveguides. A second ridge-shaped clad layer sandwiched between either one of the first clad layer or the substrate on one hand, and the first, second and third waveguides on the other hand. A diffractive grating is formed and embedded within the second clad layer corresponding to the first waveguide. A current block layer is deposited on the semiconductor substrate positioned on both sides of the longitudinal direction along the first and second clad layer and the first, second, and third waveguides. A first electrode is deposited over the first clad layer opposing to the active layer. A second electrode is spaced from the first electrode and deposited over the first clad layer opposing to the beam absorption layer. Also a third electrode is deposited on the second main surface of the semiconductor substrate. The device described above therefore is improved in the frequency-characteristics by suppressing the beam absorption and the influence of the electric field caused by the modulating signals applied to the modulator.
Further since the beam waveguide layer is formed of a bulk crystal structure, the frequency-characteristics is improved by suppressing the beam absorption and the influence of the electric field caused by the modulating signals applied to the modulator.
Further since the beam waveguide layer is formed of a multi quantum well structure, the frequency-characteristics is improved by reducing the beam absorption.
A process for manufacturing a modulator-integrated semiconductor laser device according to the present invention comprises steps as described below. A first step is preparing a layer structure including, a first waveguide of a first conductive type deposited on a first main surface of a semiconductor substrate, the first waveguide having an active layer, a first clad layer of a second conductive type semiconductor deposited on the first waveguide, and a second clad layer of a predetermined conductive type semiconductor having a diffractive grating embedded between either one of the first clad layer or the substrate and the first waveguide. A second step is forming a first stripe-shaped dielectric layer of length less than that of the substrate extending along a laser emitting direction and overlapping the diffractive grating, and etching the layer structure with using the first dielectric layer as a mask until the semiconductor substrate is exposed. A third step is forming, in sequence, a second waveguide having a beam waveguide layer of bandgap greater than that of the active layer and a third clad layer of the second conductive type semiconductor with the first dielectric layer as a mask on an etched and exposed surface of the semiconductor substrate. A fourth step is removing the first dielectric layer, forming a stripe-shaped second dielectric layer on a surface of the second and third clad layers, and the second dielectric layer extending along the laser emitting direction and covering over an portion as the first dielectric layer, and etching with the second dielectric layer as a mask until the substrate is exposed. A fifth step is forming a third waveguide and a forth clad layer of the second conductive type semiconductor on the etched and exposed surface of the substrate, the third waveguide including a beam absorption layer of semiconductor having bandgap greater than that of the active layer but less than that of the beam waveguide layer; and a sixth step of sub-steps of, removing the second dielectric layer, forming a stripe-shaped third dielectric layer on a surface of the second, third, and third clad layers, the third dielectric layer extending along the laser emitting direction and covering over an portion as the second dielectric layer, etching with using the third dielectric layer as a mask until the substrate is exposed to form a ridge, and forming a current block layer on an etched and exposed surface of the substrate. Therefore the first, second, and third waveguides are individually formed so that a high design flexibility is permitted in easily manufacturing the modulator-integrated semiconductor laser device with the improved frequency-characteristics.
A process for manufacturing a modulator-integrated semiconductor laser device comprises steps as described below. A first step is preparing a layer structure including, a first waveguide of a first conductive type deposited on a first main surface of a semiconductor substrate, the first waveguide having an active layer, a first clad layer of a second conductive type semiconductor deposited on the first waveguide, and a second clad layer of a predetermined conductive. type semiconductor having a diffractive grating embedded between either one of the first clad layer or the substrate and the first waveguide. A second step is forming a first stripe-shaped dielectric layer on the first clad layer having length less than that of the substrate extending along a laser emitting direction and overlapping the diffractive grating, and forming an opposing pair of second dielectric layers on the first clad layer defining a stripe-shaped space between them, each of the second dielectric layer extending along the laser emitting direction, the space having an end area facing to an end surface of the first dielectric layer, each of the second dielectric layer having one end adjacent to the first stripe-shaped dielectric layer and the other end, and having width narrower at the one end than that at the other end, and etching with using the first and second dielectric layers as masks until the substrate is exposed. A third step is forming by the selective growth, in sequence, a second waveguide of a semiconductor layer of bandgap greater than that of the active layer and a third clad layer of the second conductive type semiconductor with using the first and second dielectric layer as masks on an etched and exposed surface of the substrate; a fourth step including sub-steps of, removing the first and second dielectric layers, forming a stripe-shaped third dielectric layer on a surface of the second and third clad layers, the third dielectric layer extending along the laser emitting direction and covering as the second dielectric layer, etching with using the third dielectric layer as a mask until the substrate is exposed to form a ridge, and forming a current block layer on an etched and exposed substrate. Therefore the separator region waveguide and the modulator region waveguide are simultaneously formed, and steps can be simplified.
A process for manufacturing a modulator-integrated semiconductor laser device comprises steps as described below. A first step is forming an opposing air of first dielectric layers on a first main surface of a semiconductor substrate of a first conductive type, extending along a laser emitting direction and defining a stripe-shaped space between them, each of the first dielectric layer having a longitudinal direction in the laser emitting direction from one end of the semiconductor substrate to an inside position and having width narrower at the inside position than that at the one end, and forming by a selective growth a first waveguide having a first semiconductor layer of bandgap less than that of the semiconductor substrate with using the first dielectric layer as a mask until the substrate is exposed. A second step is removing the first dielectric layer, forming a second stripe-shaped dielectric layer on a surface of the first waveguide formed between the pair of first dielectric layers, having same the longitudinal length as the first dielectric layer, and etching with using the second dielectric layer as a mask until the semiconductor substrate is exposed. A third step is forming by the selective growth a second waveguide with using the second dielectric layer as a mask on an etched and exposed surface of the semiconductor substrate, the second waveguide having an active layer of bandgap less than that of the first semiconductor layer. A forth step is removing the second dielectric layer, forming a first clad layer of the second conductive type semiconductor and a beam guide layer of the second conductive type semiconductor in sequence on the first and second waveguides, etching the beam guide to form a diffractive grating opposing to the active layer, and embedding the diffractive grating within the second clad layer of the second conductive type semiconductor. A fifth step is forming a third stripe-shaped dielectric layer on the clad layer, extending along the laser emitting direction, having width narrower than that of the second dielectric layer and length covering over the first semiconductor layer and the active layer, etching with using the third dielectric layer as a mask until the semiconductor substrate is exposed to form a ridge, and forming a current block layer on an etched and exposed semiconductor substrate. Therefore the separator region waveguide and the modulator region waveguide are simultaneously formed, and steps can be simplified.
A process for manufacturing a modulator-integrated semiconductor laser device comprises steps as described below A first step is preparing a layer structure including, a first clad layer of a first conductive type semiconductor deposited on a first main surface of a semiconductor substrate of the first conductive type semiconductor, opposing to a diffractive grating extending along a laser emitting direction and having length less than that of the semiconductor substrate. A second step is forming an opposing pair of first dielectric layers on a surface of a first clad layer, extending along the laser emitting direction and defining a stripe-shaped space between them, each of the first dielectric layer having one end adjacent to the diffractive grating and the other end, and having width narrower at the one end than that at the other end, and forming by a selective growth a first waveguide with using the first dielectric layer as a mask on the substrate, the first waveguide having an active layer of bandgap less than that of the first semiconductor layer. A third step is forming a second stripe-shaped dielectric layer on a surface of the first waveguide, having a longitudinal length as the first dielectric layer formed between the pair of first dielectric layers, etching with using the second dielectric layer as a mask until the semiconductor substrate exposed; a third step for forming by the selective growth a second waveguide with using the second dielectric layer as a mask on the semiconductor substrate, the second waveguide having an active layer of bandgap less than that of the first semiconductor layer. A forth step is removing retaining dielectric layers, and forming a second clad layer on the first and second waveguides. A fifth step including sub-steps of, forming a third stripe-shaped dielectric layer on the clad layer, extending along the laser emitting direction, having width narrower than that of the second dielectric layer and length covering over the first semiconductor layer and the active layer, etching with using the third dielectric layer as a mask until the semiconductor substrate is exposed to form a ridge, and forming a current block layer on an etched and exposed semiconductor substrate. Therefore the separator region waveguide and the modulator region waveguide are simultaneously formed, and steps can be simplified.